1. Field of the Invention
The invention relates to an image forming device and particularly to the sheet transport device of the image forming device.
2. Description of Related Art
A paper feed member of this type of sheet transport device is so constructed that when two or more sheets are transported to a nip unit, one sheet is separated and transported to the downstream side in the sheet transport direction by a paper feed roll rotated to apply the force in the transport direction to the sheets transported to the nip unit and a separating member coming into contact with the transported sheets to generate the force for hindering the transport of the sheets. As the separating member brought into pressure contact with the paper feed roll, used is a roll-like member or a pad-like member having a pressure contact surface to the paper feed roll. In the case of using the roll-like member (a separation roll) as the separating member, the separation roll is rotated in the reverse direction to the sheet transport direction, thereby applying not only the force for hindering the transport of the sheets, but also the force for transporting the same in the reverse direction.
When the pressure contact force (pressure contact force of the nip unit, that is, the nip pressure) of the paper feed roll of the paper feed member and the separating member is too small or large, the transport force of the pick-up roll is too large or small, or the relationship between the transport force of the pick-up roll and the nip pressure is unsuitable, the sheets can't be surely transported one by one to the downstream side in the sheet transport direction of the paper feed member.
In the sheet transport device to which the invention applies, for example, when the contact pressure force (nip pressure) of the paper feed roll of the paper feed member and the separating member is too small, misfeed (sheet transport is not accomplished by the paper feed member) is caused. In the case where the pressure contact force is too large, when two or more sheets are simultaneously transported to the nip unit, one of the sheets can't be separated to cause multifeed of sheets (two or more sheets are transported to the downstream side without separation).
In order to prevent the occurrence of abnormal conditions in transporting a sheet, various proposals have been made heretofore, and the techniques described in the following Patent References are publicly known.
(1) Technique described in the Patent Reference 1 (Japanese Published Unexamined Patent Application No. Hei-5-32356)
According to the technique described in the patent document, a paper sheet picked up by a nudger roller (pick-up roll) is transported to a nip unit (a pressure contact area) between a feed roller (a paper feed roll) and a retard roller (a separating member) constituting a paper separating mechanism. When misfeed or multifeed is detected according to a detection signal of a misfeed detecting sensor and a multifeed detecting sensor disposed on the downstream side of the nip unit (the pressure contact area), the nip pressure (the pressure of the nip unit) is automatically controlled at any time.
(2) Technique described in the Patent Reference 2 (Japanese Published Unexamined Patent Application No. Hei-10-45272)
This Patent Reference describes a separating device including a transport roller rotated in the paper feed direction and a separation roller brought into pressure contact with the transport roller and rotated in the reverse direction to the paper feed direction. The pressure contact force of the transport roller and the separation roller is set to a small value not to feed a sheet properly at the start of feeding a sheet. The pressure contact force regulating part is controlled so that the value is gradually increased until a sheet detecting part determines the proper feeding of the sheet, and then the value at the time is kept until the separation of the sheets is completed.
(3) Technique described in the Patent Reference 3 (Japanese Published Unexamined Patent Application No. Hei-9-150990)
This Patent Reference describes the technique for transporting a paper sheet transported by a pick-up roller 20 to a pressing portion between a separation roller 21 and an opposite member 25, separating one paper sheet coming into contact with the peripheral surface of the separation roller 21, and transporting the same toward the downstream in the paper transport direction. In the technique, according to the detected speed of a paper transport speed detecting roller 41 disposed on the downstream from the separation roller 20, the pressing force P of the pick-up roller 20 and the separating force of the separation roller 21 are controlled to control the paper transport speed to a proper value, thereby normally transporting the paper sheet.
In order to normally transport the sheet as in the Patent References 1 to 3, various sheet multifeed detection methods for detecting the occurrence of abnormal sheet transport have been proposed heretofore, and the techniques described in the following Patent References (4) to (6), for example, are publicly known.
(4) Patent Reference 4 (Japanese Published Unexamined Patent Application No. Hei-11-301885)
This Patent Reference describes the technique for deciding multifeed of sheets according to the electrostatic capacity of a parallel plate electrode capacitor.
(5) Patent Reference 5 (Japanese Published Unexamined Patent Application No. 2000-095390)
This Patent Reference describes the technique for disposing an ultrasonic oscillator and a receiver on the upper side and lower side of a sheet transport path, and detecting the multifeed of sheets according to the information obtained from the oscillator and receiver.
(6) Patent Reference 6 (Japanese Published Unexamined Patent Application No. Hei-8-198478)
This Patent Reference describes the technique for detecting the multifeed of sheets according to the moving distance of a detecting roller to a transport roller when the sheet is transported.
[Patent Reference 1]: Japanese Published Unexamined Patent Application No. Hei 5-32356 (Paragraph No. [0009], FIG. 1)
[Patent Reference 2]: Japanese Published Unexamined Patent Application No. Hei 10-45272 (Paragraph No. [0015], FIG. 4)
[Patent Reference 3]: Japanese Published Unexamined Patent Application No. Hei 9-150990 (Paragraph No. [0015], [0015], [0017] to [0020], FIG. 1)
[Patent Reference 4]: Japanese Published Unexamined Patent Application No. Hei 11-301885 (Abstract on page 1)
[Patent Reference 5]: Japanese Published Unexamined Patent Application No. 2000-095390 (Abstract on page 1)
[Patent Reference 6]: Japanese Published Unexamined Patent Application No. Hei 8-198478 (Abstract on page 1)
In all techniques described in the above Patent References 4 to 6, multifeed is detected when the sheets pass through the preset position of the multifeed sensor. Accordingly, in the case where the multifeed is released by regulating the nip pressure or the like, release of multifeed can't be detected until the sheets to be separated are sent back and returned to the multifeed sensor position, resulting in causing delay in detection. On the other hand, in the case where the sheets to be separated are stopped in spite of the separating action, detection of multifeed remains as it is.